Epitope tags are short peptide sequences which can make a gene product immunoreactive to an already existing antibody and thus allow for detection, capture or purification of the gene product (Jarvik and Telmer (1998) Annu. Rev. Genet. 32:601-618). Such tags (e.g., V5, derived from a small epitope present on the P and V proteins of the paramyxovirus; c-myc, derived from a leucine zipper motif in human c-myc; and/or HA, derived from an influenza virus hemagglutinin peptide) may be derived from viral genes leading to their high immunoreactivity. Epitope tagging typically involves inserting the nucleotide sequence encoding the tag amino acid sequence into a gene of interest in frame with the coding region and expressing the gene in an appropriate host so that the tag amino acids are incorporated into the protein of interest. The tagged protein can then be detected, immobilized and/or purified by virtue of the tag's interaction with an antibody specific to the epitope tag. This approach can be used to elucidate the size of the tagged protein as well as its abundance, cellular location, posttranslational modifications and interactions with other proteins. In particular, antibodies recognizing a tag sequence facilitate detection, purification and/or isolation of tagged proteins.
Tagged proteins may include, for example, antibody molecules including antibody fragments, such as single chain antibodies (e.g., scFvs) and Fab fragments that lack a portion or all of the antibody constant domains normally found in an intact antibody. Both scFvs and Fab fragments have been successfully displayed on the surface of viruses and cells such as bacteriophage and bacterial or yeast cells, which has allowed for the selection of antibody fragments with antigen binding affinities as high as their divalent counterparts. Evaluation of antibodies is greatly facilitated by immobilization or capture of the antibody and analysis of its interaction with other proteins. Full-length IgGs may be captured through their Fc region using an anti-Fc antibody or by binding to proteins such as Protein A, Protein G, Protein A/G or other antibody binding proteins. However, the lack of an Fc domain in some antibody fragments, such as single chain antibodies (e.g., ScFv) or single domain antibodies (e.g., dAb), precludes the use of an Fc capture reagent. While Fab fragments contain a CH1/CL constant region, this domain varies between different subclasses of antibodies and this does not provide a region that allows consistent capture with single Fc capture reagent. In addition, capturing with anti-Fab antibodies may interfere with binding of the antibody to its antigen. As such, tag sequences have been employed to capture antibody binding fragments that lack an Fc region in whole or in part.